This invention relates to a system for analyzing the performance of a gas turbine engine. More particularly, this invention relates to a system for monitoring a gas turbine engine wherein various engine performance parameters are sensed and compared with predicted values of the same parameters to supply deviation or error signals indicative of the condition of the engine.
In the gas turbine engine arts, the need for apparatus and methods for monitoring the performance of a gas turbine engine to provide diagnostic information such as the detection of an engine fault and provide prognostic information such as the time the engine can be operated until maintenance procedures are required has long been recognized. The need for such apparatus and techniques is especially apparent with respect to gas turbine engines which power aircraft since the failure of these engines often causes delayed departures and maintenance problems which are not only costly to the aircraft operator, but, in the case of commercial transport aircraft, cause inconvenience and potential economic loss to passengers and the shippers of freight. Further, the present practice of overhauling aircraft gas turbine engines after an empirical period of operation is not cost effective in that, although such practice tends to minimize the probability of inservice engine failure, many engines are prematurely removed from service for maintenance or overhaul. In addition, it has been recognized that a gas turbine monitoring system which provides an immediate indication of engine malfunction or of impending failure will allow an aircraft crew to take immediate action which can minimize damage to the engine and further enhance aircraft safety margins.
Accordingly, a variety of systems have been proposed to monitor and analyze the operation of a gas turbine engine in a manner which detects engine malfunction or failure and/or provides diagnostic or prognostic information that is of value in engine maintenance and overhaul operations. Basically, these prior art monitoring systems can be classified as either ground based systems which utilize engine data gathered during previous flights of the aircraft or as airborne systems which either continuously or periodically sense various engine performance parameters.
Generally, the ground based systems include equipment on the aircraft to periodically record the values of various engine performance parameters during periods of aircraft operation in which the monitored engine is operated under predetermined conditions and include a ground based monitoring station which generally utilizes a large scale digital computer. In the operation of such a system, recorded engine performance data is either transmitted to the ground based monitoring station or, more typically, stored on magnetic tape or other media within the aircraft for delivery to the ground station at a later convenient time. Once the engine data is received at the ground station, it is generally "conditioned" by filtering techniques to remove a substantial portion of the noise content and to normalize the data so that it is amenable to processing within the particular computer and analysis routine that is employed. After such conditioning and normalization, the data is stored within a data bank for later computer processing. Generally, this processing is performed on a periodic basis to determine the temporal or trend characteristics of the monitored engine parameters, which trend information is useful in supplementing periodical overhaul policies to prevent premature removal of an engine. Additionally, when an engine fails in service, the previously recorded engine performance data can be processed within the computer to aid in determining the cause of engine failure and hence ensure that adequate overhaul procedures are followed before the engine is returned to service.
Although such ground based engine monitoring systems have both diagnostic and prognostic capabilities, several disadvantages and drawbacks are presented. First, and foremost, the analysis of gas turbine engine performance is not effected as the engine operates (i.e., in "real time" or "on-line") but, because of delays encountered in recording the engine data, transmitting it to the ground station and analyzing the data within the computer, the diagnostic and prognostic capabilities of the system are limited. For example, such "after-the-fact" analysis may not be readily available when an engine must be removed and quickly restored to service. Further, such after-the-fact analysis does not provide the flight crew with current engine performance data and in-flight procedures which could prevent serious engine damage cannot be initiated. In addition, although such a ground based station can serve a number of aircraft, the costs associated with operating such a system are relatively high both in terms of the required investment in equipment and the labor costs involved in operating and maintaining such a system.
Previously proposed gas turbine engine monitoring systems that are operable in an airborne application are typified by U.S. Pat. No. 3,238,768, which issued to D. V. Richardson on March 8, 1966 and U.S. Pat. No. 3,731,070, which issued to Lewis A. Urban on May 1, 1973. Both of these systems are based on the concept that the thermodynamic processes occurring within a gas turbine engine can at least be approximated by one or more functions of various engine performance parameters. In this respect, the systems disclosed by these references can be said to incorporate system modeling techniques or electronic simulation.
For example, in the system disclosed by Richardson, the engine pressure ratio (EPR) of a twin spool gas turbine engine is monitored and, in the mathematical sense, is treated as an independent variable that is used to determine values of fuel flow (W), exhaust gas temperature (EGT) and the rotational speed of the high pressure compressor stage (N2) which would result in a properly operating theoretical or ideal engine of the type being monitored. More specifically, in this arrangement, pressure sensors are utilized to determine the pressure at the inlet of the high pressure compressor stage (P2) and the pressure at the inlet of the engine exhaust nozzle (P7). The ratio of the signals supplied by these sensors (P7/P2) is equal to the EPR and is supplied to three diode networks that are arranged to effect voltage transfer characteristics that closely approximate the parametric relationship between the independent variable (EPR) and one of the associated dependent variables (W, EGT, and N2) in an "ideal" engine of the type being monitored. Thus, the signals supplied by the diode networks are intended to represent expected values of W, EGT, and N2 that would result if the engine being monitored exactly corresponds to such a "ideal" engine, and if the monitored engine is operating properly. To detect that the monitored engine is not performing as expected, the signals supplied by the diode networks and signals representative of the actual value of the engine parameters (W, EGT and N2) are supplied to analog computational networks which calculate the deviation between the expected and actual value of each of these parameters.
In monitoring systems of the type disclosed in the previously referenced patent to Urban, sensors detect the values of various engine performance parameters, and reference or base line information that represents the parametric relationship between a number of engine performance parameters which are utilized as dependent variables and an engine performance parameter such as EPR is stored within the memory of a special purpose computer when the system is first installed. During operation of this system, the difference between the actual value of each sensed performance parameter (dependent variable) and the corresponding stored information is determined by the computer to supply a set of parameter deviation signals. The set of deviation signals is then utilized in the computer along with a stored set of coefficients to compute deviations in a set of independent engine performance parameters whose values ideally vary only with degradation in engine performance. The values of coefficients utilized are dictated by the type of engine being monitored and are the coefficents of a set of linear differential equations which characterize the interrelationship between changes in the monitored engine performance parameters (dependent variables) and changes in the engine performance parameters which constitute the associated independent variables.
When the system computer determines that a deviation has occurred in one or more of the calculated independent variables, signals representative of these deviations are logically combined to activate fault indicators that indicate which engine components should be inspected to locate the fault. Additionally, in the system disclosed by the Urban patent, the computed deviations in the independent variables are plotted as a function of time for comparison with predetermined limits to prognosticate the future operating condition of the monitored gas turbine engine.
Various disadvantages and drawbacks are encountered with prior art airborne gas turbine engine monitoring systems of the above described types. For example, neither of these systems is capable of precisely monitoring engine performance over the entire operating regime of the engine being monitored. In this respect, since the base line information that is gathered and stored in the type of system disclosed by the Urban patent necessarily reflects a single engine operating condition (e.g., static sea level operation), and since in actual practice, the values of engine performance parameters do not remain constant under other operating conditions (e.g., operation of the aircraft at other altitudes and speeds), such a system would appear to be inherently limited to utilization with the monitored engine being operated under conditions which correspond to those conditions under which the base line information was recorded. Alternatively, if such a system is utilized under other engine operating conditions, compromises in system accuracy must be accepted. Similarly, since a system of the type disclosed by the Richardson patent depends on preascertained relationships to characterize the performance of a theoretical nominal engine of the type being monitored, it would appear that such a system cannot compensate for, or accomodate, natural variations in engine performance parameters that arise under various engine operating conditions. Further, even though gas turbine engines are manufactured and refurbished in accordance with strict dimensional tolerances, the actual performance parameters of engines within a particular type deviate from theoretical or nominal values by an amount that is significant with respect to the accuracy requirements of a monitoring system which will detect minute changes in engine performance parameters to thereby indicate engine deterioration before serious engine failure occurs. Thus, most prior art gas turbine engine monitoring systems which rely on a theoretical engine model are inherently limited in monitoring accuracy. Systems such as those disclosed by the Urban patent at least partially overcome this limitation by utilizing previously recorded data that is obtained from the engine being monitored as the engine monitoring reference or base line. However, attaining a high degree of accuracy in such a system not only requires that the differential equations which model or simulate the thermodynamic characteristics of the type of engine being monitored precisely describe such thermodynamic characteristics, but also requires that each engine of a particular type exhibit exactly the same characteristics. Since structural variations do occur because of manufacturing and installation tolerances and since gradual deterioration in performance occurs throughout an engine's operating life, the constant coefficient modeling approach utilized in the system disclosed by the Urban patent cannot provide the degree of accuracy that is necessary or desired in exacting engine monitoring applications.
In addition, neither type of the previously described prior art systems addresses the problem of sensor inaccuracies or signal noise which is commonly encountered with commercially available pressure and temperature sensors. In this respect, in a system such as that disclosed in the Urban patent, wherein system accuracy depends on the validity of recorded base line data, precisely defining the base line information would generally require several measurements at each operating point to obtain a statistically valid mean value. This procedure would generally require a fairly sophisticated digital computer having a nonvolatile memory that is capable of storing the large amount of collected data throughout the operational life of the monitored engine. Since such computers are not only economically unattractive for airborne engine monitoring applications, but also increase the weight and structural complexity of the monitoring system, a highly accurate system of the type disclosed in the Urban patent may not be practical or desirable.
Accordingly, it is an object of this invention to provide a highly accurate system for real-time monitoring of the performance parameters of a gas turbine engine wherein system accuracy is not substantially affected by normal variations associated with engine manufacturing, installation tolerances and/or normal engine deterioration.
It is another object of this invention to provide a system for monitoring the performance parameters of a gas turbine engine in which the system is accurate over a wide range of engine operating conditions.
It is yet another object of this invention to provide a system for monitoring the performance parameters of a gas turbine engine wherein noise and inaccuracies associated with the system sensors have minimal effect on the system accuracy and reliability.
Still further, it is an object of this invention to provide a performance monitoring system of the above-described type which exhibits a degree of structural and computational simplicity that is necessary and desirable for real-time airborne monitoring applications.